equip SMARTData with a validity check

this fixes vbox drives getting reported as possibly breaking when indeed
they simply lack any amount of smart data. currently I have no evidence
that this is a more wide spread issue beyond vbox so the validity
condition is fairly constrained.

this also sees a slight refactor of smartctl which previously had error
some weak failure handling, by moving that outside the kauth-dependent
code we can actually test this though and smartmonitor is easily retro
fitted for validity handling anyway